1. Field of the Invention
The present invention relates generally to integrated circuit packaging, and more particularly to a thermally enhanced packaging system for thin film integrated circuits.
2. Background Art
An integrated circuit dissipates power primarily in the form of heat. Typical semiconductor devices have an ambient operating temperature range from 0 to 70 degrees Celsius, although some devices have ambient operating temperatures beyond this range. The temperature of the semiconductor substrate depends upon the heat generating characteristics of the integrated circuit components, the heat dispersive properties of the packaging, and the ambient temperature of the surrounding environment. As the temperature of the semiconductor substrate approaches the high temperature operating limit of the semiconductor device, the performance of the semiconductor device substantially degrades. High temperature operation of a semiconductor device reduces its operating lifespan, causes it to operate at slower speeds, and causes it to display other non-ideal operating characteristics.
Semiconductor devices are often required to dissipate significant amounts of power to perform useful functions. Power, dissipated in the form of heat, travels by conduction through the semiconductor substrate into a plastic molding or other case or packaging enclosing the device. As advances in integration techniques continue to reduce the physical size of semiconductor devices, the size of integrated circuit packaging is reduced as well. Smaller enclosures have less mass to absorb heat and less surface to radiate heat away from the semiconductor device. Thus higher circuit integration requires greater relative power dissipation from smaller integrated circuit packages.
Heat dissipation from integrated circuit packages can be enhanced by means of externally mounted metallic heat sinks. Such heat sinks are typically made of aluminum or aluminum alloy and are attached to the top surface of a molded plastic integrated circuit package by mechanical means or with a thermally conductive adhesive.
Although externally mounted heat sinks can significantly enhance the heat dissipation of an integrated circuit package, external heat sinks are not suitable for many applications. External heat sinks substantially increase the dimensions of an integrated circuit package. In applications where components are closely aligned or the system chassis is closely fitted, external heat sinks complicate design efforts and often result in wasted space.
Heat dissipation from an integrated circuit package may also be increased by internal means. The bulk of an integrated circuit package consists of plastic, a thermal insulator. The heat dissipated by the integrated circuit chip encapsulated in the molded plastic package can be enhanced by providing additional thermal pathways from the integrated circuit chip to the ambient environment.
A standard, non-thermally enhanced molded plastic integrated circuit package is shown in FIGS. 1, 2 and 3. As shown in FIG. 1, integrated circuit package 105 consists of a molded plastic body 100 containing an integrated circuit die 110 mounted to a die pad 120, a leadframe 130, and bond wires 140 electrically connecting die 110 to the leads of leadframe 130. A plan view of die pad 120 and leadframe 130 is shown in FIG. 2. As shown in FIG. 2, die pad 120 consists of a rectangular substrate. Leadframe 130 consists of a series conductive strips extending radially outward adjacent to die pad 120. Leadframe 130 and die pad 120 are separated by a gap 200 around the perimeter of die pad 120. Gap 200 is also shown in FIG. 1. As shown in FIG. 1, bond wires 140 form an electrical connection across gap 200 between die 110 and the individual leads making up leadframe 130. Bond wires 140 are usually very thin. Accordingly, although they conduct heat as well as electricity, because of their small diameter, their thermal resistivity is relatively high.
FIG. 3 is a top view of integrated circuit package 105. FIG. 3 shows how the ends of the individual leads 300 of leadframe 130 extend from integrated circuit package 105. Leads 300 form the pins used to make electrical connections from the integrated circuit to external circuitry. As shown in FIG. 1, leads 300 typically have a downset 135. Downset is a dogleg bend that brings ends 145 of individual leads 300 below the level of the bottom of molded body 100 to facilitate mounting of integrated circuit package 105 to a printed circuit board.
FIG. 4 illustrates an integrated circuit package 405 with an integrated heat sink 410. As shown in FIG. 4, the lower half of integrated circuit package 405 includes a molded plastic body 400 containing a die 110, lead frame 130, and bond wires 140. Integrated circuit package 405 does not include a die pad. The upper part of integrated circuit package 405 incorporates a heat sink 410 separated by double sided, thermally conductive/electrically insulative tape 415 from die 110. Integrated circuit package 405 provides greater heat dissipation than standard integrated circuit package 105. However, integrated circuit package 405 is significantly more difficult and costly to produce. Because the leadframe and heat sink have to be tooled separately, the cost of integrated circuit package 405 is three to five times that of unenhanced integrated circuit package 105.
FIG. 5 illustrates an integrated circuit package with an internal heat spreader. As shown in FIG. 5, this type of integrated circuit package 505 contains an internal heat spreader 500 in addition to die 110, die pad 120, leadframe 130, bond wire 140 and package body 100. Internal heat spreader 500 is typically made of aluminum and may or may not be in thermal contact with the leadframe. In the package shown in FIG. 5, heat spreader 500 is not in contact with leadframe 130.
Heat spreader 500 has "legs" 510 on which it stands when placed into a mold. As plastic mold compound is injected into the mold, it raises heat spreader 500 such that it contacts the underside of die pad 120. In the finished package, mold body 100 completely encompasses heat spreader 500.
Internal heat spreaders such as heat spreader 500 are used in the thicker integrated circuit packages such as plastic leadless chip carriers (PLCC's) and plastic dual in-line pin packages (PDIP's).
Internal heat spreader 500 exhibits some of the same disadvantages as an external heat sink. Because heat spreader 500 is encapsulated in plastic molded body 100, it is insulated from the ambient air by the molded body 100's high thermal resistivity. The high resistivity of molded body 100 prevents heat from being dissipated effectively. In addition, because heat spreader 405 is contained entirely within package 505, package 505 is thicker than a similar package that does not include a heat spreader.
Integrated circuit packages incorporating heat spreaders in thermal contact with the lead frame are shown in FIGS. 6 and 7.
Integrated circuit package 605 shown in FIG. 6, like integrated circuit package 505 of FIG. 5, consists of molded body 100, die 110, die pad 120, leadframe 130, and bond wires 140. Integrated circuit package 605 also contains a heat spreader 600 that extends beyond die pad 120 such that it is in thermal contact with leadframe 130. Double sided thermally conductive/electrically insulative tape 610 is disposed between heat spreader 600 and leadframe 130 to allow heat conduction but provide electrical insulation between heat spreader 600 and leadframe 130.
Integrated circuit package 705 shown in FIG. 7 is similar to integrated circuit package 605 of FIG. 6, incorporating a molded body 100, die 110, leadframe 130, bond wires 140, heat spreader 700 and double sided thermally conductive/electrically insulative tape 710 disposed between heat spreader 700 and leadframe 130. However, integrated circuit package 705 does not include a die pad 120. Instead, die 110 is supported by heat spreader 700.
While heat spreader integrated circuit packages 605 and 705 need not be as thick as drop-in heat spreader integrated circuit package 505, they are more expensive to produce, costing one and one half to two times as much as unenhanced integrated circuit package 105.
FIG. 8 illustrates an integrated circuit package 805 incorporating a "bat-wing" leadframe 810. As shown in FIG. 8, in a bat-wing leadframe integrated circuit package, the thermal path between die pad 120 and one or more leads 815 is enhanced by a bat-wing shaped conducting surface 820 that forms an enlarged thermal path between die pad 120 and the upper ends 825 of leads 815. In FIG. 8 molded package body 100 has been cut away to expose conducting surface 820. In the actual package, molded body 100 encapsulates and covers conducting surface 820. Accordingly, conducting surface 820 provides increased heat conduction only within molded body 100 from die pad 120 to the upper ends 825 of leads 815. The remainder of the thermal path from upper ends 825 of leads 815 to the tips of leads 815 is restricted by the smaller cross section of the leads 815 themselves. Accordingly, heat conduction to a circuit board via leads 815 and from there to the ambient environment remains limited.
There remains a need for an integrated circuit package that provides enhanced heat dissipation with little or no added cost.